Today there is an increasing demand for low cost finger print sensor devices for biometric authentication based on finger print recognition. However, since the cost of a finger print sensor often is proportional to the area of the sensor, and since the area of the sensor needs to be comparable to the dimensions of the finger, it is not trivial to bring down the cost of silicon based finger print sensors sufficiently; this is true especially for area sensors.
However, one way to solve this is to implement the detection electrodes in a separate and completely or at least partly passive Sensor Substrate, which can be made out of other materials and processes than single-crystalline silicon needed for electronic circuits. In this way a potential lower-cost of such a passive or partly passive Sensor Substrate can be utilized for obtaining an overall cost-efficient solution combining a low cost Sensor Substrate with a relatively smaller electronic (interface) circuit.
In such a configuration the Sensor Substrate can have a size matching the dimensions of the finger and it is this possible to combine this with an Electronic Circuit of significantly smaller dimensions and correspondingly lower cost than what would be the cost of an Electronic Circuit with essentially the same dimensions as the finger.
However, although the overall cost of the components of such a finger print sensor, consisting of a Sensor Substrate and an Electronic Circuit can be significantly lower than a corresponding solution based on a single-chip solution, the cost of assembling the two parts will increase the cost of the finished finger print sensor, in spite of the continuously reduced cost levels of such assembly processes in volume manufacture of today.
One important cost factor is related to the electric connections between such a Sensor Substrate and the Electronic Circuit. The cost of such interface connections is normally proportional to the actual number of interface connections. One way to reduce the cost of the interface connections is thus to reduce the number of interface signals.
Additionally, on the electronic circuit there is also a cost associated with every interface signal, since every signal requires a minimum connection area, which normally cannot be utilized for other functions. Thus if the number of interface signals between the electronic circuit and the Sensor Substrate can be reduced, the cost of the electronic circuit can also be reduced.